The hot-dip production of steel coated with zinc, zinc alloy or iron-zinc alloy or of annealed galvanised steel, called galvannealed steel, has been known for years. The obvious interest in these products in the automotive industry has led to high standards in terms of homogeneity and performance properties of the product in the use of these coated products.
In a hot-dip galvanisation line, the strip emerging at the outlet of the zinc bath is dried using “air knives”, which blow compressed air onto the molten zinc taken along by each side of the strip. The pressure of the air knives and the distance between the knife and the strip should ideally be adjusted in real time in such a way that the thickness of the deposit is maintained as close as possible to the desired value on each side of the strip.
With a view to ensuring uniform coating, it is essential to maintain correct shape and position of the strip in motion opposite the lips of the drying apparatus. Indeed, variations in the position of the strip in the direction transverse to the motion direction opposite the knives will produce variation in the thickness of the deposit. These fluctuations in thickness affect the quality of the end product. In particular, in the galvannealing process, variations in the weight of zinc inevitably lead to variations in the iron content of the deposit and consequently to heterogeneity in the properties required by the user.
Owing to thermal and mechanical stresses, instability in the air pressure or structural vibration however, the flatness of the strip moving in front of the lips is not perfect. Significant differences in the thickness of the deposit are observed in the transverse and longitudinal directions.
Owing to the variation in the weight of coating during processing, it is necessary to establish a sufficiently high set point value with a view to obtaining the minimum weight of deposit required by the consumer.
Let us calculate the benefit that a reduction of 1 g/m2 in the weight of the zinc coating would produce.
Let T be the number of tonnes produced per year, Em the average thickness of the strip (in m), P the price per kg of zinc and D the relative density of steel (in kg/m3).
The result is:
Zinc saved (tonnes/year)2T/(D × Em × 1000)Zinc saved (kg/tonne of steel)2/(D × Em)Money saved (ε/tonne of steel)2 × P/(D × Em)Money saved (ε/year)2 × P × T(D × Em)
For example, if T=350,000 tonnes/year, Em=0.0007 m, D=7,800 kg/m3 and P=0.85 /kg (1050 $/tonne), the following savings will be obtained through a reduction of 1 g/m2:    Zinc saved: 128 tonnes of zinc/year    Zinc saved: 0.366 kg of zinc/tonne of steel    Money saved: 0.31 /tonne of steel or 108.5 thousand /year.
These data show that it is worthwhile searching solutions that are capable of increasing the control of the coating thickness.